Linearity is becoming increasingly important in mixed-signal systems such as radio frequency (RF) communications, radar, Electronic Warfare (EW), Signal Intelligence (SIGINT), and sensor systems. Driving the need for high linearity is the increasing use of digital beamforming phased array and frequency channelized configurations. FIG. 1 shows an example of a digital beamforming receiver system 10 having a plurality of receive channels. Each receive channel includes an antenna 12 (or group of antennas called a sub-array), an RF receiver 14, and an ADC (Analog-to-Digital Converter) 16. The RF receiver 14 of a given channel converts incoming RF signals to an intermediate frequency (IF). The ADC 16 samples the IF and produces an ADC channel output 18. A digital beamformer 20 receives the ADC channel outputs 18 from the multiple channels and computes a digital beam 22 by multiplying each channel by beamforming weights and computing sums.
In a single channel system, the RF receiver 14 and ADC 16 are generally designed such that the spurs and intermods 26 are below the noise level 24 at the channel output 18 of the ADC so that they do not interfere with detection of small signals (only intermods are shown in this output 18). However, in the digital beamforming system, the digital beamforming process enhances the signal-to-noise ratio (SNR) because the signals add coherently and noises add in power. The spurs and intermods 26 also tend to add coherently during the digital beamforming process because they tend to be highly correlated from channel to channel. Generally, the SNR gain is approximately 3 log2N (dB), where N represents the number of channels being combined. Therefore, after digital beamforming, the spurs and intermod levels 26 can finish above the noise level 24′ if many channels are combined to provide high SNR gain as shown in the digital beam 22. These spurs and intermods 26 can interfere with the small signal detection, thus demonstrating the importance of linearity processes in the phased array receivers that can attenuation such nonlinear distortions.
FIG. 2 shows an example of a frequency channelized receiver system 30, capable of achieving similar SNR gain to that of the digital beamforming system 10 of FIG. 1, and further illustrating the importance of linearity. The frequency channelized receiver system 30 includes an antenna 32, an RF receiver 34, an ADC 36, and a frequency channelizer 38. The RF receiver 34 converts incoming RF signals from the antenna 32 to an intermediate frequency (IF). The ADC 36 samples the IF and produces an ADC digitized output 40. The digitized output 40 has a two-tone input signal 42, an initial noise level 44, and spurs and intermods 46, which are lower in power than the noise level 44. To compute the sub-band in the digital frequency channelization process, the frequency channelizer 38 bandpass filters the wideband signal to produce narrowband signals (output 48). During the bandpass filtering process, the out-of-band noise is filtered out and only in-band noise 50 remains. Therefore, if the in-band noise is spread evenly across all frequencies, the overall noise power is reduced as shown in the “equivalent noise level” 52, and the SNR in the sub-band ends up higher than the initial full-band SNR. The SNR gain is approximately 3 log2M (dB) where M is the number of sub-bands. Therefore, the in-band spurs and intermods 46 that were initially below the noise level 44 can finish above the noise level 52 after the digital channelization process. These spurs and intermods 46 can interfere with small signal detection.
Some communication and sensor systems have both digital beamforming and digital frequency channelization. In such systems, the SNR gain is equivalent to 3 log2MN (dB). With such high SNR gain, even low level spurs and intermods can interfere with small signal detection, again illustrating the importance of linearizing the ADC response.